The instant invention relates to an optical spectrophotometer having a light source which emits optical radiation (IR, visible, and UV), and a monochromator which isolates narrow portions of the spectrum of the emitted light in order to measure the absorption or emission bands of a substance.
Such an optical spectrophotometer can be used to detect the presence of substances in a medium, in particular, to measure the concentration of pollutants in the air.
An optical spectrophotometer of this type is described in U.S. Pat. No. 4,732,476. It includes a monochromator provided with a diffraction grating which rotates around a predetermined axis at a constant angular velocity. When the diffraction grating is illuminated with white light, it produces in a testing chamber a monochromatic light ray having a wavelength which goes through the entire spectrum as a function of the position of the diffraction grating and of time. A photodetector is located after the testing chamber and measures the intensity of the spectrum altered by the absorption effect of substances which are present in the testing chamber.
Another spectrophotometer is known from U.S. Pat. No. 4,070,111 in which a diffraction grating is seated on the axle of a galvanometer. Filtered light, the wavelength of which scans back and forth across the spectrum of the light source, is broken up for analysis into a reference ray and into a measured ray passing through a testing chamber.
Further, GB-A 2,181,536 discloses an optical sensing arrangement in which a testing chamber for gaseous substances is irradiated with light coming from a white light source. The spectrum of the light which is altered by the substances present in the testing chamber is analyzed in a Fabry-Perot resonator which acts as a filter with a narrow bandwidth. The optical length of the Fabry-Perot resonator is varied periodically by means of an electric signal having an asymmetric saw-tooth configuration. During every period, the Fabry-Perot resonator is caused to scan a narrow portion of the spectrum that is determined by the electric signal. The thus-filtered light reaches the photodetector and produces an output signal which is evaluated as a function of the electric signal.
It is the object of the instant invention to provide an inexpensive optical spectrophotometer which is of simple construction and which makes it possible to amplify a measured signal with little noise.